Terbium based detector scintillator

ABSTRACT

An imaging system ( 100 ) includes a radiation source ( 110 ) and a radiation sensitive detector array ( 116 ), which includes a scintillator array ( 118 ) and a photosensor array ( 120 ) optically coupled to the scintillator array, wherein the scintillator array includes Gd 2 O 2 S:Pr,Tb,Ce. A method includes detecting radiation with a radiation sensitive detector array ( 116 ) of an imaging system ( 100 ), wherein the radiation sensitive detector array includes a Gd 2 O 2 S:Pr,Tb,Ce based scintillator array ( 118 ). A radiation sensitive detector array ( 116 ) includes a scintillator array ( 118 ) and a photosensor array ( 120 ) optically coupled to the scintillator array, wherein the scintillator array includes Gd 2 O 2 S:Pr,Tb,Ce, and an amount of Tb 3+  in the Gd 2 O 2 S:Pr,Tb,Ce is equal to or less than two hundred mole parts per million.

FIELD OF THE INVENTION

The following generally relates to a radiation sensitive imagingdetector with a terbium (Tb³⁺) based scintillator array and is describedwith particular application to computed tomography (CT). However, thefollowing is also applicable to other imaging modalities.

BACKGROUND OF THE INVENTION

A computer tomography (CT) scanner includes an x-ray tube and a detectorarray. The x-ray tube is supported by a rotating gantry, which rotatesabout an examination region, thereby rotating the x-ray tube about theexamination region. A detector array is located opposite the x-ray tube,across the examination region. The x-ray tube emits radiation thattraverses the examination region (and a portion of a subject or objecttherein) and illuminates the detector array. The detector array detectsradiation traversing the examination region and generates a signalindicative thereof. A reconstructor reconstructs the signal, generatingthree dimensional volumetric imaging data. A data processor can processthe three dimensional volumetric imaging data and generate one or moreimages based thereon.

A conventional detector array has included a scintillator based detectorarray. A typical scintillator based detector array includes ascintillator array optically coupled to a photodiode array. By way ofexample, a conventional scintillator based detector array has included agadolinium oxysulfide (GOS) based (e.g., Gd₂O₂S:Pr,Ce) scintillatorarray optically coupled to a silicon (Si) photodiode array. Theradiation traversing the examination region illuminates the scintillatorarray, which absorbs the x-ray photons and, in response, emits opticalphotons, which are indicative of the absorbed x-ray photons. Thephotodiode array detects the optical photons and generates an electricalsignal indicative of the detected optical photons. The reconstructorreconstructs this signal.

Gd₂O₂S:Pr,Ce based scintillator arrays have had a light yield or outputof about 40,000 photons/MeV, with an afterglow suitable for CTapplications. Generally, the light output corresponds to conversionefficiency, or the ability to convert absorbed x-ray photons intooptical photons. Thus, there is an unresolved need for scintillatorarrays with higher conversion efficiency and light output, withafterglow suitable for CT applications.

SUMMARY OF THE INVENTION

Aspects of the present application address the above-referenced mattersand others.

According to one aspect, an imaging system includes a radiation sourceand a radiation sensitive detector array, which includes a scintillatorarray and a photosensor array optically coupled to the scintillatorarray, wherein the scintillator array includes Gd₂O₂S:Pr,Tb,Ce.

According to another aspect, a method includes detecting radiation witha radiation sensitive detector array of an imaging system, wherein theradiation sensitive detector array includes a Gd₂O₂S:Pr,Tb,Ce basedscintillator array (118)s.

According to another aspect, a radiation sensitive detector arrayincludes a scintillator array and a photosensor array optically coupledto the scintillator array, wherein the scintillator array includesGd₂O₂S:Pr,Tb,Ce, and an amount of Tb³⁺ in the Gd₂O₂S:Pr,Tb,Ce is equalto or less than two hundred parts per million.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

FIG. 1 schematically illustrates an example imaging system with adetector array including a terbium based scintillator array.

FIGS. 2, 3, and 4 graphically illustrate time dependent emissionintensity curves of Gd₂O₂S :Pr,Tb,Ce after x-ray excitation respectivelyfor three different amounts of Tb³⁺ in connection with a predeterminedlight output threshold of 200 ppm and a predetermined threshold time of5 ms.

FIGS. 5, 6, and 7 graphically illustrate time dependent emissionintensity curves of Gd₂O₂S :Pr,Tb,Ce after x-ray excitation respectivelyfor three different amounts of Tb³⁺ in connection with predeterminedlight output thresholds of 200 and 50 ppm and predetermined thresholdtimes of 5 and 500 ms.

FIG. 8 illustrates an example method for detecting a presence of Tb³⁺ ina scintillator.

FIG. 9 illustrates an example imaging method employing a detector arraywith a scintillator array having Gd₂O₂S:Pr,Tb,Ce.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates an imaging system 100 such as a computed tomography(CT) scanner.

The imaging system 100 includes a stationary gantry 102 and a rotatinggantry 104, which is rotatably supported by the stationary gantry 102.The rotating gantry 104 rotates around an examination region 106 about alongitudinal or z-axis 108.

A radiation source 110, such as an x-ray tube, is supported by androtates with the rotating gantry 104, and emits radiation towards theexamination region 106.

A source collimator 112 collimates the emitted radiation to form agenerally cone, fan, wedge, or otherwise shaped radiation beam thattraverses the examination region 106 and a portion of an object orsubject therein.

A radiation sensitive detector array 114 is affixed to the rotatinggantry 104 and subtends an angular arc, across from the radiation source110, opposite the examination region 106. The illustrated detector array114 includes at least one detector module 116 with a scintillator array118 optically coupled to a photosensor array 120. The scintillator array118 absorbs x-ray photons 122 and, in response, emits optical photons(e.g., visible light or ultraviolet radiation), which are indicative ofthe absorbed x-ray photons 122. The photosensor array 120 detects theoptical photons and generates an electrical (current or voltage) signalindicative of the detected optical photons.

As described in greater detail below, the illustrated scintillator array118 includes Gd₂O₂S:Pr,Tb,Ce in which the amount of terbium (Tb³⁺) inthe Gd₂O₂S:Pr,Tb,Ce increases light output (i.e., photon-to-lightconversion efficiency), relative to a configuration without the terbium(Tb³⁺), while satisfying a predetermined afterglow (or light decay)threshold. In the illustrated embodiment, the photosensor array 120 iscoupled to a back of the scintillator array 118. In another embodiment,the photosensor array 120 is coupled to a side of the scintillator array118. In addition, suitable scintillator arrays include composite andceramic scintillator array, such as those respectively described inUS201000032578 and US2010/00167909, which are incorporated in theirentirety by reference herein.

A reconstructor 124 reconstructs the signal and generates volumetricimage data indicative of the examination region 106 and the portion ofthe subject or object therein.

A subject support 126, such as a couch, supports an object or subject inthe examination region 106. The support 126 is movable along the x, yand z-axes in coordination with the rotation of the rotating gantry 104to facilitate helical, axial, or other desired scanning trajectories.

A general purpose computing system serves as an operator console 128,which includes human readable output devices such as a display and inputdevices such as a keyboard and/or mouse. Software resident on theconsole 128 allows the operator to control an operation of the system100, for example, by allowing the operator to initiate scanning, etc.

As briefly discussed above, the illustrated scintillator array 118 isco-doped with an amount of Tb³⁺ which increases light output, relativeto a configuration without the Tb³⁺, while satisfying a predeterminedafterglow threshold.

FIGS. 2, 3 and 4 graphically illustrate time dependent emissionintensity curves 200, 300 and 400 of Gd₂O₂S:Pr,Tb,Ce after x-rayexcitation is stopped respectively for three different amounts of Tb³⁺.In FIGS. 2, 3 and 4, a y-axis 202 represents normalized light intensityin a logarithmic scale and an x-axis 204 represents time in alogarithmic scale. For these examples, a light output threshold 206 isset at 200 parts per million (ppm) at a time threshold 208 of 5milliseconds (ms). In other embodiments, the thresholds 206 and/or 208can be can different, including larger or smaller.

In FIG. 2, the amount of Tb³⁺ in the Gd₂O₂S:Pr,Tb,Ce is approximately 10mole ppm, in FIG. 3, the amount of Tb³⁺ in the Gd₂O₂S:Pr,Tb,Ce isapproximately 50 mole ppm, and in FIG. 4, the amount of Tb³⁺ in theGd₂O₂S:Pr,Tb,Ce is approximately 200 mole ppm. From FIGS. 2, 3 and 4,the light output threshold 206 is satisfied by all three amounts of Tb³⁺(10, 50 and 200 ppm) at the time threshold 208, as shown by the curves200, 300 and 400, as the curves 200, 300 and 400 all fall at or belowthe light output threshold 206 at an intersection 210 of the lightoutput threshold 206 and at the time threshold 208.

The above is summarized in TABLE 1 below.

TABLE 1 Light yield for a given amount of Tb³⁺ at 5 ms. Figure Tb³⁺(mole Light yield Normalized afterglow Number ppm) (photon/MeV) (ppm) at5 ms FIG. 1 10 46,700 100 FIG. 2 50 49,200 100 FIG. 3 200 53,000 200

From FIGS. 2, 3 and 4 and TABLE 1, for the light output threshold 206 of200 ppm, an amount of Tb³⁺ up to 200 mole ppm can be used. With thisamount, the light output of the scintillator array 118 is approximately53,000 photon/MeV. This represents an increase of light output ofapproximately 33% relative to the 40,000 photon/MeV light output of thescintillator discussed in the background. Higher increases in lightyield, for example, by more than 35% such as up to 50%, can be achievedwhile still complying with CT time dependent light intensityspecifications. For amounts of Tb³⁺ over 200 ppm, the Tb³⁺ begins todominate the afterglow behavior, increasing the effective shortafterglow of the scintillator, due to the relatively slow emission ofTb³⁺ in Gd₂O₂S.

Also from FIGS. 2, 3 and 4 and TABLE 1, where the light output threshold206 is instead 100 ppm or less, an amount of Tb³⁺ up to 50 mole ppm canbe used. With this amount, the light output of the scintillator array118 is approximately 49,200 photon/MeV. This represents an increase oflight output of approximately 23% relative to the 40,000 photon/MeVlight output of the scintillator discussed in the background. From FIGS.2, 3 and 4 and TABLE 1, where the light output threshold 206 is greaterthan 200 ppm, an amount of Tb³⁺ greater than 200 mole ppm can be used.

Generally, as Gd₂O₂S:Pr,Ce does not show losses due to concentrationquenching nor to thermal quenching of the luminescence, very likely thetransfer of energy from host lattice states to Pr³⁺ states hasefficiency smaller than unity, consequently this energy is not used togenerate light to be used subsequently in the CT procedures. WithGd₂O₂S:Pr,Tb,Ce, the additional amount of Tb³⁺ provides an additionalradiative recombination or energy scavenging channel. However, as Tb³⁺has a much slower intrinsic decay time, this sets an upper limit to theTb³⁺ concentration. Too high a Tb³⁺ concentrations results in too highrelative light intensities as a function of time, compared to the lightintensity immediately after the X-ray pulse as only a fraction of thephotons is generated by Tb³⁺ ions, of course also Pr³⁺ ions contributeto the photon generation process. So the relative contributions of Tb³⁺and Pr³⁺ have to be tuned carefully so as not to degrade properties ofGd₂O₂S:Pr,Ce, such as the very small afterglow.

FIGS. 2, 3, and 4 include a single threshold point, namely, theintersection 210, as criteria to determine a maximum amount of Tb³⁺which can be added to the scintillator material to increase lightoutput. It is to be appreciated that the amount of Tb³⁺ to add can bedetermined based on more than a single threshold point. An example ofthis is shown in connection with FIGS. 5, 6, and 7. Similar to FIGS. 2,3 and 4, in FIGS. 5, 6, and 7 the y-axis 202 represents normalized lightintensity in a logarithmic scale and the x-axis 204 represents time in alogarithmic scale.

From FIGS. 5, 6 and 7, a second light output threshold 502 of 50 ppm issatisfied by all three amounts of Tb³⁺ (10, 50 and 200 mole ppm) at asecond time threshold 504 of 500 ms, as shown by the curves 200, 300 and400, as the curves 200, 300 and 400 all fall below the second lightoutput threshold 502 at an intersection 506 of the second light outputthreshold 502 and at the second time threshold 504. In other examples,still more light output and/or time thresholds can be used, whereappropriate.

FIG. 8 illustrates a method for detecting a presence of Tb³⁺in ascintillator.

It is to be appreciated that the ordering of the following acts is forillustrative purposes and not limiting. As such, the ordering may bedifferent, including concurrent acts. Moreover, one or more of the actscan be omitted and/or one or more acts can be included.

At 802, a light source is activated to illuminate the scintillator array118. An example of a suitable light source includes a light source thatemits 254 nm light.

At 804, the scintillator array 118 is excited by the illumination.

At 806, the scintillator array 118, in response to being illuminated,emits characteristic radiation.

At 808, the emission spectrum of the emitted radiation is measured.

At 810, the measured emission spectrum is analyzed to determine whetherTb³⁺ is present. In one instance, this includes identifying a presenceof emissions lines below 490 nm, where Pr³⁺ does not emit in thismaterial, and which are indicative of a presence of Tb³⁺. Such linesinclude lines at about 450 nm, 410 nm and 380 nm. A high emissionintensity line at 545 nm may also be observed. Other approaches are alsocontemplated herein.

FIG. 9 illustrates an imaging method.

It is to be appreciated that the ordering of the follow acts is forillustrative purposes and not limiting. As such, the ordering may bedifferent, including concurrent acts. Moreover, one or more of the actscan be omitted and/or one or more acts can be included.

At 902, radiation is produced and emitted by the radiation source 110.

At 904, the emitted radiation traverses the examination region 106 and aportion of a subject or object therein.

At 906, radiation traversing the examination region 106 and the portionof the subject or object therein is received by the scintillator array118, which absorbs the radiation and emits optical photons indicative ofthe received radiation.

As discussed herein, in one instance, the scintillator 118 includes anamount of Tb³⁺ to achieve a desired light output with a desired lightdecay. For example, as shown herein, for a light output of less than 200ppm at or after 5 ms, less than 200 ppm of Tb³⁺ can be used to increasethe light intensity by approximately 33% relative to a configuration inwhich the Tb³⁺ is not added.

At 908, the optical photons are detected via the photosensor array 120,which generates an electrical signal indicative of the detectedradiation.

At 910, the electrical signal is reconstructed, thereby generatingvolumetric image data indicative of the examination region 106 and theportion of the subject or object therein.

The above may be implemented via one or more processors executing one ormore computer readable instructions encoded or embodied on computerreadable storage medium such as physical memory which causes the one ormore processors to carry out the various acts and/or other functionsand/or acts. Additionally or alternatively, the one or more processorscan execute instructions carried by transitory medium such as a signalor carrier wave.

Although the above describes determining the amount Tb for ascintillator based on a combination of light output and decay timethresholds, in another instance, the amount Tb is based on apredetermined ratio of Tb and Pr, Tb and Ce, Tb and Pr and Ce, and/or Tbto other elements of the scintillator. In yet another instance, theamount Tb is based solely on a predetermined light output threshold. Theapproximate relative Light Yield (LY) (e.g., a number between 0 and 1)can be determine by: [N(Pr)+25N(Tb)]/[N(Pr)+25N(Tb)+25N(Ce)+400], whereN is in mole ppm. The absolute light yield is then given by:45000*[N(Pr)+25N(Tb)]/[N(Pr)+25N(Tb)+25N(Ce)+400]. In still anotherinstance, the amount Tb is based on another predeterminedcharacteristic.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. An imaging system, comprising; a radiation source; and a radiationsensitive detector array, including; a scintillator array; and aphotosensor array optically coupled to the scintillator array, whereinthe scintillator array includes Gd₂O₂S:Pr,Tb,Ce, wherein the amount ofthe Tb³⁺ is equal to or less than fifty mole parts per million.
 2. Theimaging system of claim 1, wherein the Gd₂O₂S:Pr,Tb,Ce includes Tb³⁺ inan amount in which a light output of the scintillator is below apredetermined light output threshold at a predetermined decay time. 3.The imaging system of claim 2, wherein the amount of the Tb³⁺ is equalto or less than ten mole parts per million. 4-5. (canceled)
 6. Theimaging system of claim 1, wherein the light output is approximately53,000 photon/MeV.
 7. The imaging system of claim 1, wherein thescintillator array includes a composite material.
 8. The imaging systemof claim 1, wherein the scintillator array includes a ceramic material.9. The imaging system of claim 1, wherein a light efficiency of thescintillator array is approximately thirty three percent greater than aconfiguration in which the scintillator array does not include the Tb³⁺.10. The imaging system of claim 1, wherein the imaging system is acomputed tomography scanner.
 11. A method, comprising: detectingradiation with a radiation sensitive detector array of an imagingsystem, wherein the radiation sensitive detector array includes aGd₂O₂S:Pr,Tb,Ce based scintillator array and the amount of the Tb³⁺ isequal to or less than ten mole parts per million.
 12. The method ofclaim 11, wherein the Gd₂O₂S:Pr,Tb,Ce includes Tb³⁺ in an amount inwhich a light output of the scintillator is below a predetermined lightoutput threshold at a predetermined decay time.
 13. The method of claim12, wherein the amount of the Tb³⁺ is equal to or less than ten moleparts per million. 14-15. (canceled)
 16. The method of claim 11, whereinthe light output is approximately 46,700 photon/MeV.
 17. The method ofclaim 11, wherein the scintillator array includes one of a compositematerial or a ceramic material.
 18. The method of claim 11, wherein alight efficiency of the scintillator array is approximately thirty threepercent greater than a configuration in which the scintillator arraydoes not include the Tb³⁺.
 19. The method of claim 11, wherein theimaging system is a computed tomography scanner.
 20. A radiationsensitive detector array, comprising: scintillator array; and aphotosensor array optically coupled to the scintillator array, whereinthe scintillator array includes Gd₂O₂S:Pr,Tb,Ce, and an amount of Tb³⁺in the Gd₂O₂S:Pr,Tb,Ce is equal to or less than ten mole parts permillion.
 21. The detector array of claim 20, wherein Gd₂O₂S:Pr,Tb,Ceincludes Tb³⁺ in an amount in which a light output of the scintillatoris below a predetermined light output threshold at a predetermined decaytime.
 22. The detector array of claim 20, wherein the scintillator arrayincludes a composite material or a ceramic material.
 23. The detectorarray of claim 20, wherein a light efficiency of the scintillator arrayis approximately thirty three percent greater than a configuration inwhich the scintillator array does not include the Tb³⁺.
 24. The detectorarray of claim 20, wherein the imaging system is a computed tomographyscanner.